NCIS has said goodbye to many beloved characters over the years.

Sadly, the series will have to say goodbye to Dr. Donald "Ducky" Mallard following David McCallum's death in September.

CBS and those close to the show have yet to reveal whether a tribute episode is in the works, but there's no way the series will proceed without a fitting tribute to an actor who brought so much to the show.

For a series that's been on the air for 20 years, there has been a lot of cast turnover, meaning the show has an excellent opportunity to bring back some people who have been long gone to help say goodbye to Ducky.

But who should return for the big event?

Mark Harmon as Leroy Jethro Gibbs

Harmon's time at the wheel concluded with NCIS Season 19, but Gibbs seemed happy with his new outlook on life.

He left his team in the trusty hands of Gary Cole's Agent Alden Parker, but it always seemed like a temporary hiatus to keep Gibbs in the teams' orbit if something happened.

Gibbs and Ducky have had a great relationship since NCIS Season 1, so we imagine that Harmon will be high on producers' wish lists to stage a comeback.

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For his part, Harmon hasn't ruled out returning, and we imagine it would be fun to return for an episode here and there as opposed to being locked into 20+ episode seasons.

Broadcast TV has the tightest production schedules, and it must be exhausting.

Cote de Pablo as Ziva David

There's a reason why fans clamored for de Pablo's return when we were led to believe that Ziva had been killed off-screen: She's one of the best characters in the franchise.

Ziva also had many memorable encounters with Ducky while on the team.

It would make perfect sense for her to want to pay her respects to such a cherished member of her former team.

The good news? We know Ziva's alive and well after de Pablo staged a short-term comeback as part of one of the most shocking storylines in NCIS history.

Michael Weatherley as Tony DiNozzo

Weatherley was one of the most popular actors on the show for 13 seasons, and even years later, fans have been bellowing for his return.

For his part, Weatherley has been teasing a comeback left, right, and center, but we won't believe it's happening until CBS confirms it.

There is still some uncharted territory for Tony and Ziva, so it wouldn't surprise us if Weatherley returned alongside de Pablo to pay tribute to Ducky.

In the process, fans can learn what the couple has been up to in the years since their supposed reunion.

Pauley Perrette as Abby Sciuto

Abby had a fantastic bond with Ducky, but there's a high chance Perette will not be interested in returning to the series.

Perrette departed during NCIS Season 15 and has been vocal countless times about not being interested in returning.

Perrette and Harmon had a massive falling out, and it doesn't seem like something they've worked through.

The only chance I see of the actress returning is if Harmon declines to return for the episode, as well.

Another possibility involves Abby appearing via a video message.

We'll need to wait for some updates on NCIS Season 21 to find out what's happening on that front.

Emily Wickersham as Ellie

Wickersham's departure from NCIS Season 18 remains one of the series' biggest mysteries.

It came out of nowhere and left us with more questions than answers.

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Ellie returning from her top-secret mission would undoubtedly leave fans begging for her to share the screen with Wilmer Valderamma's Nick Torres.

The series seemed to set us up for another NCIS romance for the ages, but it wasn't meant to be.

Sadly, we don't think a one-episode stint would bring that storyline to a close.

But Ellie was a big part of the team for several seasons, so bringing her back would be a good idea.

Whether Wickersham is ready to return is unclear, but we'll eagerly await answers.

Maria Bello as Dr. Jack Sloane

Maria Bello's three-season run on the series wrapped in March 2021.

Fans connected with Jack Sloane in a big way, so a return as she pays tribute to Ducky, a man with whom she had a storied history, would be interesting.

It's also worth noting that Sloane had quite the connection with Gibbs, and we wouldn't mind seeing them revisit it.

Many other faces could be seen again, but the above six are the people whose returns we believe would have the most significant impact.

Those six returning and their myriad of connections to other characters might leave newer team members out in the cold, or it could provide a cherished bridge between the old and the new that viewers will never forget.

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Given that the series has been on the air for two decades, there are many connections between various team members, but the stars would need to align to bring them to life again.

Schedules are a factor, and given that NCIS and most other broadcast shows were delayed due to the dual Hollywood strikes, the series is facing an accelerated production schedule to get new episodes on the air early next year.

That means that schedules won't be as malleable as before, and some people who are interested in revisiting NCIS may not be able to stage a comeback.

The important thing is that the inevitable tribute to Ducky is befitting of the character, the actor who portrayed him, and his many contributions; the show will never be the same without him.

What are your thoughts on who should return for a tribute episode?

Hit the comments below.

NCIS Season 21 is set to premiere on Monday, February 12, 2024.

Paul Dailly was an Associate Editor for TV Fanatic.Follow him on X.